This invention relates to flaw detection, characterization and studying by acoustic imaging and has particular relationship to such flaw detection and characterization where the surface of the work or specimen under observation is irregular.
Flaw detection by acoustic imaging is used in the inspection of nuclear reactor pressure vessels. In this area acoustic imaging helps to characterize the flaws or defects precisely enough so that, by fracture mechanics, the defects can be reliably estimated. Nuclear reactor pressure vessels are clad with stainless steel or other usually ferrous anti-corrosion material. The cladding is usually deposited by arc welding and the surface of the cladding is irregular. The irregularities vary the acoustic path of the energy which is propagated during the imaging and deteriorate the reliability of the results.
As disclosed in Hurwitz application, in acoustic imaging, acoustic energy from an acoustic lens or a focussed-arc transducer submerged in a liquid, typically water, is propagated to the work and scans the work through the liquid. The resulting acoustic-energy echoes received from defects of flaws are processed electronically and optically to produce an acoustic pattern from which the flaws may be characterized. This invention predominately concerns itself with acoustic imaging in which acoustic energy is focussed on or near the surface of the work by an acoustic-lens transducer and produces or stores a holographic pattern. Where clad work is inspected, the acoustic energy is focussed on or near the irregular surface. This invention is uniquely applicable to holographic presentation but in its ramifications it may be applicable to other forms of presentations. To the extent that this invention is so applicable, such use is within the scope of this application or of any patent which may issue on or as a result thereof.
For an understanding of the influence, on acoustic imaging, of irregularities in the surface of the work impinged by the acoustic energy, it is necessary that the effect of the irregularities on the acoustic path length be determined. If this effect were negligible, no problem would be confronted. In fact, under the conditions under which acoustic imaging is carried out, the effect is appreciable and may at times be devastating.
Typically, the frequency of the acoustic energy is 4 megahertz for which the wavelength .lambda..sub.1 =0.015 inch in water. The liquid through which the propagation of acoustic energy takes place is assumed to be water. The ratio of the sound velocity in alloys such as stainless steel to that in water is 4:1. As in optical imaging, acoustic imaging can tolerate random phase errors of only 1/8 to 1/4 the wavelength. Because of the high ratio of sound velocity and because of the short wavelength of the acoustic energy, small irregularities in the surface of the work can drastically change the acoustic-path length, in terms of wavelengths, over different parts of the scanned raster.
For acoustic energy impinging on a water-steel interface at normal incidence, the acoustic path-length P in numbers of wavelength is, to a first approximation, defined by the equation: ##EQU1## where:
D.sub.1 is the actual distance traversed by the acoustic energy in water;
D.sub.2 is the actual distance traversed by the acoustic energy in steel;
.lambda..sub.1 is the wavelength of the acoustic energy in water; and
.lambda..sub.2 is the wavelength of the acoustic energy in steel.
Let D=D.sub.1 +D.sub.2 ; D is a constant. ##EQU2## where:
n.sub.1 is the index of refraction in water;
n.sub.2 is the index of refraction in steel. ##EQU3## where .DELTA.P is an increment in path length produced by an increment in travel distance in medium 1, the water.
For a water to steel interface; using a frequency of 4 MHz EQU n.sub.2 =0.25, .lambda..sub.1 =0.015.
For an allowable path length error of 1/4 wavelength, equation (5) becomes ##EQU4##
It appears then, in the case of a randomly irregular surface, that if the root-mean-square random ripple of the surface (D.sub.1) is of the order of 0.005 inch, the acoustic images are destroyed. If the surface irregularities are systematic, for example of spherical or cylindrical curvature or in steps, larger errors than 1/4 wavelength can be tolerated. Such irregularities produce correctable distortions but do not destroy the images. In fact, random fluctuations whose roughness is about 10 times the 0.005 permissible roughness occur in the surface of the cladding for some nuclear reactor vessels.
It is an object of this invention to produce clear, well-defined, undistorted images, particularly holographic reconstructed images, by acoustic imaging to achieve reliable detection and characterization of flaws in work such as the cladding of nuclear reactor vessels, notwithstanding the irregularity of the surface scanned by the acoustic energy.